1. Field of the Invention
The present invention relates to papermaking machines and, more specifically, to an apparatus for conditioning a fabric and associated system and method.
2. Description of Related Art
A through-air drying (TAD) fabric is typically an open-weave fabric configured to receive a relatively wet paper web and to transport the web to one or more drying devices, such as a through-air dryer, for the drying device to interact with the fabric and the web to form a relatively dry paper web. The web is then separated from the fabric downstream of the drying device and advanced to subsequent processes. However, the paper web may undesirably leave residue on the fabric after being separated therefrom. Since the fabric is typically configured as a loop, the residue must be removed from the fabric, before the fabric advances back to the point at which the web is received by the fabric, in order to, for example, avoid contamination of the paper web. In addition, the residue on the fabric may also reduce the permeability thereof and may adversely affect drying performance, resulting in “wet spots” in the web as the web exits the drying device(s). In light of this requirement, the fabric is usually cleaned with water and then dewatered, in a collective process known as conditioning, when the fabric is not carrying the web.
In one known conditioning method, characterized as a horizontal conditioning configuration as shown in FIG. 1, the fabric 5 enters the conditioning section 10 and wraps about an inlet roll 15. The fabric 5 includes an outer or sheet side 5A for carrying the web, and an opposing inner side 5B. At the upstream side of the inlet roll 15, the fabric 5 is subjected to cleaning fluid, such as water, from a flooded nip shower 20. As the fabric 5 further advances about the inlet roll 15, the fabric 5 may also be subjected to cleaning fluid from one or more fan or needle showers 25, 30, 35 that may direct the cleaning fluid at the fabric 5 at different angles and toward one or both sides of the fabric 5. A variety of shower types and shower positions may be implemented depending on process requirements. A vacuum box 40 is disposed downstream of the showers for removing excess cleaning fluid from the fabric 5, in a process referred to as dewatering. In some instances, other devices, such as an air knife or another vacuum device, may be used in the alternative or in addition to the vacuum box 40 for dewatering the fabric 5. As such, according to this method, the fabric 5 is directed to run in a generally horizontal orientation to the inlet roll 15 and, after advancing over the inlet roll 15, in the opposite direction in a generally horizontal orientation to pass the showers and the vacuum box 40. Thereafter, the fabric 5 is advanced about an exit roll 45 so as to direct the fabric 5 back toward the point at which the web is received and, as a result, is generally directed under the showers dispensing the cleaning fluid and the drainage system for collecting the excess cleaning fluid as well as the cleaning fluid dewatered from the fabric 5.
In another known conditioning method, characterized as a vertical conditioning configuration as shown in FIG. 2, the fabric 5 is advanced over the inlet roll 15 and then advanced downward in a generally vertical direction. A flooded nip shower 20 is directed at the fabric 5 and disposed at the upstream side of the inlet roll 15. On the downward run, the fabric 5 is subjected to one or more showers, wherein, as before, various shower types and shower positions may be implemented depending on process requirements. Following the showers, the fabric 5 is subject to one or more dewatering devices, such as a vacuum box 40 or air knife. The fabric 5 is then directed about an exit roll 45 and otherwise redirected back to the point at which the web is received.
The known conditioning methods, however, may have characteristics that can have an adverse effect on the papermaking process. For example, the water or cleaning fluid emitted by the showers in spray or solid stream form tends to bypass the vacuum box or other dewatering device and undesirably rewet the fabric before exiting the conditioning section. In order to address this rewetting problem, elaborate and often expensive sealing methods may be employed. However, these sealing methods are often ineffective and may require extensive maintenance. Further, since no removal of the cleaning fluid from the fabric, other than by limited drainage due to gravity, is performed before fabric is subjected to dewatering by the vacuum box or other dewatering device, the fabric typically arrives at the vacuum box in a very wet condition. As such, in order for the vacuum box to effectively remove the sufficient and/or desired amount of water from the fabric, the vacuum box must generally be operated at a high level, which may cause an undesirably high level of power consumption.
In some instances, where the fabric comprises a through-air drying (TAD) fabric, the outer side of the TAD fabric may be sanded or otherwise processed (such as by hot calendering) in order to achieve a desired contact area between the web formed thereon and downstream processes. Sanding of the TAD fabric provides for particular contact characteristics between the fabric and the web, as well as between the web and downstream processes. For example, the fabric may be sanded to achieve about a 20% contact area between the web and the cylinder of a Yankee dryer, as the web is transferred thereto from the drying fabric. As such, the condition of the sanded surface of the fabric may affect some parameters, such as the style or weave pattern imparted to the web, the coarseness of the web, or the contact area between the web and the cylinder of a Yankee dryer, and thereby affect the properties of the web. In such instances of a sanded fabric surface, contact between the sanded outer side of the advancing fabric and certain stationary elements may cause wear to the sanded outer side and, as a result, may adversely increase the contact area of the fabric and thereby detrimentally affect the applicable process parameters. In this regard, the known conditioning methods illustrated in FIGS. 1 and 2 both indicate that the vacuum box used for dewatering the fabric is configured to interact with the sheet or outer side of the fabric that may be sanded.
In the event of operational problems within the drying section, including a Yankee dryer, if provided, there may be instances in which the web may not be removed or separated from the fabric prior to the fabric entering the conditioning section. As a result, a full width sheet of the web may sometimes inadvertently enter the conditioning section. Where the dryer comprises a through-air dryer (TAD), the drying conditions of the TAD tend to cure a high wet strength resin to a greater extent than other drying methods and, as such, a high wet strength resin may be difficult to dissolve under the conditions present in the conditioning section. In addition, some of the liquid extracted from the web/fabric by the molding box and/or transfer device may also tend to include such a high wet strength resin, which may further tend to remain with or adhere to the fabric during the drying process. Accordingly, a conditioning section should desirably be configured to allow ready access for removing any portions of the paper web, or associated fines or chemicals, that are not removed from the fabric in normal operating conditions. The conditioning section should also be configured to facilitate periodic maintenance, as well as fabric changing procedures.
Thus, with respect to a conditioning section in a drying section of a papermaking machine, there exists a need for a fabric-conditioning apparatus and method that reduces or eliminates rewetting of the fabric exiting the conditioning process. Such a solution should desirably be accomplished without requiring elaborate and expensive sealing provisions. The conditioning apparatus and method should also desirably reduce or eliminate contact between outer side of the fabric and stationary elements. There also exists a need for an effective dewatering process with reduced energy consumption.